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Hellenistic science
The Hellenistic Age, which began with Macedonian king Alexander the Great's creation of an empire encompassing the area between Greece and India, was a time that saw the cross-fertilization of cultures, the rise of powerful independent monarchies, and important scientific innovations. Though much of what is known of those scientific discoveries has been filtered by later authors in the ancient world, what is known is impressive. The third-century-BCE astronomer Aristarchus of Samos, for example, hypothesized a heliocentric (i.e., sun-centered) universe, an idea that was not revisited—or accepted—until after the time of Nicholas Copernicus nearly 2,000 years later. While Hellenistic astronomers, mathematicians, doctors, and engineers built upon the ideas of earlier thinkers, they nonetheless broke ground that their forefathers had not. One reason for that was that many of those scientists and philosophers had the appropriate leisure time and the financial backing of rich kings to investigate their interests. In addition, the extension of Greek culture into new lands that began with Alexander also brought the intellectual wealth of other civilizations, especially from Persia and Egypt, into wider currency.
In the field of astronomy, Aristarchus of Samos, Hipparchus of Bithynia, and Egypt's Ptolemy claim pride of place. Aristarchus was the first to posit a heliocentric model of the universe. His work on that theory has not survived, but subsequent thinkers, notably Greece's Archimedes, mention his ideas. According to Archimedes' The Sand-Reckoner, a largely mathematical work that discusses some astronomical matters, Aristarchus claimed that the sun and the "fixed-stars" did not move. That was contrary to the prevailing theory, which posited that the stars (and the sun) were embedded in invisible crystalline spheres that rotated around the Earth. In Aristarchus' model, the Earth revolves around the sun. The heliocentric theory failed to impress his contemporaries, some of whom, like Hipparchus, devised theories that seemed to support the geocentric (Earth-centered) model of the universe.
Hipparchus contributed to the epicyclic model of planetary motion in his theory for the movement of the sun and moon (that complex theory explained, among other things, why objects sometimes look as if they move faster when close to Earth and slower when far away). To further his own observations, Hipparchus used the epicyclic model and Babylonian records of eclipses (which contained information reaching back centuries) to calculate estimates for the motion of the sun and moon. His most important contribution to astronomy was his work on the procession of the equinoxes, in which he charted the points along the Earth's orbit around the sun. The conclusions Hipparchus reached were very close to those of astronomers today. While technically a figure of the post-Hellenistic period, Ptolemy, who became the most influential astronomer of the ancient world, took the epicyclic theory and posited a geocentric model that lasted until the scientific revolution that flourished in the 17th century CE.
Many of the period's innovations in mathematics were as much astronomical as they were mathematical. For example, in the third century BCE, Eratosthenes of Cyrene calculated the circumference of the Earth and was fairly accurate. He assumed that the sun's beams were roughly parallel, and thus that if he measured the angle at which they hit the Earth at two locations and knew the distance between the two locations, he could calculate the Earth's circumference. On the summer solstice, Eratosthenes compared the angle of a stick's shadow at Alexandria and Syene. Using geometry, he then calculated the Earth's circumference. Among his many other contributions, including works on geography, astronomy, drama, and history, Eratosthenes also worked on prime numbers, geometry, and mathematics in music. Known as "Beta" (the second letter of the Greek alphabet), Eratosthenes was regarded as being facile in a wide variety of fields while being expert in none.
More so than any other mathematician, Euclid has justly been cited as the prime example of Hellenistic innovation in mathematics. In the third century BCE, Euclid built on the solid traditions of past mathematicians and is best known for his book The Elements. The Elements covered plane geometry, number theory, irrationals, and solid geometry. Important to his theories was a belief that certain basic principles, or axioms, helped in treating mathematical problems deductively. His ideas were the foundation of Western geometry for 2,000 years.
Medicine in the ancient world advanced more in the Hellenistic period than it had for centuries. Alexandria, Egypt, where the monarchs of the Ptolemaic dynasty were great patrons of learning (the museum and the famous Library of Alexandria were constructed under Ptolemy I Soter and Ptolemy II Philadelphus), was one of the main centers for medical study. Erasistratus of Ceos and his colleague Herophilus were two of the third century BCE's most prominent medical men. Although primarily a philosopher of medicine, Erasistratus did investigate human anatomy and observed that motor and sensory nerves are different. He and Herophilus conducted dissections on executed prisoners (and, according to Aulus Cornelius Celsus, a later medical man, even on live ones) and discovered, via comparison, the differences in healthy and diseased organs. Digestion, the circulatory system, and growth all interested Erasistratus. Herophilus contributed to ancient knowledge about the brain, eyes, and liver. He was also one of the first to attribute illness to an imbalance of humors. Like the four elements, there were four humors: blood, phlegm, yellow bile, and black bile. When the four humors were in balance, one was healthy, but when they were out of balance, one became sick. Both Erasistratus and Herophilus recommended rest and special diets. While the various schools of medicine in the Hellenistic period eventually declined, much of the medical theory of the Hellenistic period influenced Galen of Pergamum, perhaps the best known medical thinker of the ancient world, centuries later during the second century CE.
The Hellenistic period also witnessed some advancements in the field of engineering, particularly in the military sphere. Few sources survive, but those that do suggest that scholars were as interested in engineering as they were in other scientific fields. Many of the devices common in modern times, like the lever and the pulley, were already in use by 300 BCE, but there were some new mechanisms that became very important. Archimedes, perhaps the best engineering mind of the third century BCE, invented a screw that was vital in irrigation for lifting water. He was also said to have invented various siege machines that his native city used against the Romans during the second siege of Syracuse. Prior to that, Ctesibius of Alexander invented devices that used air as an air pump and improved the water clock. Many of the period's engineering feats are most clearly visible in the new and improved artillery weapons of the Hellenistic kings. For example, Alexander the Great's engineers designed better siege towers and missile devices.
The Hellenistic period gave rise to many of the theories that dominated Western science for nearly two millennia. The Ptolemaic universe (which drew heavily upon the work of Hellenistic astronomers); the geometry of Euclid; the advances in anatomical knowledge; and the improvements to machinery, both for warfare and labor, all became foundational to later science and in some cases (notably with Euclid) are still a part of school curricula today. Much of the science that saw advancements in the 18th, 19th, and early 20th centuries used concepts and machines that were invented in the Hellenistic Age.